Tony Harrison doesn't shirk the big issues. His new epic verse play deals with environmental disaster, the precariousness of human survival, the failure of idealism, and the saving power of imagination. It hardly makes for a coherent whole, but it has exciting moments and a wild madcap inventiveness.

The structure is odd. The Greek scholar, Gilbert Murray, rises from Westminster Abbey plaque to present, with the aid of Sybil Thorndike, a drama called Fram: the name of the ship of Fridtjof Nansen. But later the Arctic explorer won the Nobel Peace Prize - for his attempt to relieve Russian famine - and was a driving force behind the highly idealistic League of Nations.

Nansen underwent a conversion from "a Darwinian with the emphasis on win", to a philanthropist who sought to warn about global hardship and ecological doom. But Harrison is less interested in the transformation than in its consequences. The question asked is whether attitudes can best be changed by charitable enterprise, international institutions, or art and language.

It is an interesting dialectic. Easily the best moment comes when Sian Thomas's Thorndike delivers a speech about starvation that brings home the reality of the Volga famine. It is a tour de force by Thomas and shows Harrison's language at its best. But he later uses the ghost of Nansen's polar companion to pour scorn on the inflated claims of art, and suggests western civilisation will be powerless against future disasters, famines and wars. One can't expect Harrison to resolve the art versus life debate, but a coda rather confusingly implies that, with greater empathy and more contact between opposed value-systems, mankind stands a chance.

He has bitten off more than any single play can chew and, dramatically, there are dead patches. But I can forgive any play that aims high. And there is abundant theatricality in a production, by Harrison himself and his formidable designer Bob Crowley, that embraces intellectual argument and classical ballet and that is written in exuberant rhyming couplets. Jasper Britton as the life-changing Nansen, Jeff Rawle as the Eliot-hating Murray and Thomas as an unexpectedly svelte Sybil also give high-quality performances.